With regard to natural food, the amateur should take care to ensure a good stock for the young fish. Many of the creatures suitable for food may be cultivated in separate ponds at the same time as the fish, if a natural supply is not at hand. The Daphnia pulex (water flea) and the Cyclops quadricornis may be introduced into the boxes very soon after the fish have began to feed. Daphnia breeds at the rate which is almost inconceivable. The female produces her first brood of young when she is ten days old, and goes on breeding at an average of three or four times a month. The female and her progeny are rendered fertile by one act of coition, probably for fifteen generations at least, without any further intervention of the male. Both Daphnia and Cyclops are bred in stagnant water in which there should be a good stock of weeds.
The fresh water shrimp (Gammarus pulex) is an excellent form of food for young and old trout, and should be given to the fry as soon as they are old enough to manage them. Corixæ and other small insects should also be given as often as possible. The fresh-water shrimp is bred in running water, Corixæ in still or slow running water. Weeds are necessary to the well-being of both.
The boxes must be kept carefully covered, as I have already pointed out. A kingfisher would make short work of a box of fry, and other birds and beasts of various kinds are partial to them. There are only two courses open to the fish culturist in dealing with these enemies—to protect his fish or kill the enemies. I prefer to protect the fish first and kill the enemies afterwards.
The greatest care must be taken not to introduce, or allow to intrude, any water beetles or the larger carnivorous aquatic larvæ of insects, into the rearing boxes. I have known cases where the larvæ of the Dytiscus marginalis, the largest of our carnivorous water beetles, have destroyed almost all the fry in a rearing pond. The adult D. marginalis itself is not a whit less voracious, and much stronger than its larva.
If the wooden parts of the apparatus have been properly prepared, according to my previous instructions, there should be no risk of the fry developing fungus. Quite a small spot of woodwork, however, left uncovered by asphalt-varnish, or enamel, or uncharred, will render the chance of the development of this disease probable.
Should by any misfortune fungus get into the rearing boxes, a dose of salt may very likely cure it. Sea water is the best, but if this is not obtainable, a solution of salt and water run through the boxes will probably cure the disease. Considerable good may also be done to the young fish by occasionally putting a lump of rock salt in at the inlet, and the water allowed to run over and dissolve it.
CHAPTER VIII
TROUT. THE MANAGEMENT OF THE FRY (Continued)
In the last chapter I brought my reader up to the point where the fry, which had been feeding for some time in the rearing boxes, had been judiciously separated, the weaker and smaller fish which took up their positions at the lower ends of the boxes having been put into separate boxes and induced as much as possible to keep at the head near to where the current enters.
It is difficult to lay down any certain rule as to what is the best time at which to take the next step—that of turning the fry out into the rearing ponds. When the fry have got into more or less regular habits, and showing no fear of whoever it is who feeds them, come up readily and seize the food boldly, is probably the best time to let them out into the larger space of the pond. I do not mean to say that when a certain proportion of the fish have got over their natural shyness, and feed boldly and without hesitation, the whole of them should be set free. What I mean is, that when the habit of associating the appearance of a certain individual with a meal has been well established among them for a week or so, they should be allowed to escape from the box into the pond.